In a typical spectrophotometer, the target wavelength or wavelength range is automatically set or a preset wavelength range is automatically scanned by a computer-controlled motor. This automatic setting and scanning requires not only a mechanism for rotating a spectral element (such as a diffraction grating or a prism) with high precision but some other mechanisms working in conjunction with the spectral element rotating mechanism.
A spectrophotometer for use in the range covering the ultraviolet light and visible light normally includes two light sources, one for visible light and the other for ultraviolet light. In such a spectrophotometer, a light source switching mechanism is used to select and place either of the two light sources on an optical path according to the target wavelength or wavelength scanning range. The light source switching mechanism is realized by rotating a converging mirror or by shifting the light sources.
When a diffraction grating is used as the spectral element, the optical system of a spectrophotometer is designed so that only (positive or negative) first order diffraction light having the greatest diffraction intensity is emitted through the exit slit. The principle of the diffraction grating, however, also allows the exit slit to emit second order or higher order diffraction light, as well as the first order light. Ordinary spectrophotometers thus use a filter for eliminating unnecessary second or higher order diffraction light. In many spectrophotometers, several high-pass (for eliminating light of shorter wavelength) or band-pass colored glass filters are provided, and one of them is selected and placed in the optical path by a filter selecting mechanism depending on the wavelength of the operated light. The filter selecting mechanism is disposed before or after the diffraction grating.
These three driving mechanisms, that is, a) one for rotating the diffraction grating, b) one for switching the two light sources, and c) one for selecting the filter, are essential for a spectrophotometer that covers visible and ultraviolet range and automatically sets and scans a target wavelength or wavelength range. These driving mechanisms usually use stepping motors as the driving source because stepping motors are relatively small, easily controlled by a microcomputer, and can rotate at low speed without reduction gears. In a spectrophotometer, stepping motors of the driving mechanisms cooperate interlockingly with one another according to the target wavelength or scanning wavelengths under the control of a microcomputer. The stepping motors are usually controlled in an open-loop system.
FIG. 9 shows a typical prior art spectrophotometer that uses a swinging converging-mirror 3 for switching two light sources and a concave diffraction grating 9 as the spectral element. The converging mirror 3 is swung by a light source switching mechanism 4 to select either an ultraviolet light source 1 or a visible light source 2. A filter selecting mechanism 7 is provided for placing an appropriate filter 6 for cutting off higher order light in the optical path. The diffraction grating 9 is rotated by a diffraction grating rotating mechanism 10. White light coming through an entrance slit 13 is reflected, diffracted and converged by the concave diffraction grating 9, and a monochromatic image of the entrance slit 13 is focused on an exit slit 14, where the wavelength of the image on the exit slit 14 depends on the position of the diffraction grating 9. The monochromatic light emitted through the exit slit 14 passes through a sample 16, is focused on a photometer 12 by an optical system 15, and is measured by the photometer 12.
In normal operations, the stepping motors used as the driving sources of the mechanisms 4, 7, and 10 can be correctly controlled in an open-loop system without using a feed-back sensor. But, at the moment when the spectrophotometer is switched on and power supply to the motors are started, the rotational position of the stepping motors are indefinite: so are the position of the converging mirror 3, the position of the diffraction grating 9, and the kind of filter 6 placed in the optical path. Then it is impossible for the controller (microcomputer) of the spectrophotometer to control the stepping motors correctly according to a preset program unless the operation origins of the motors are determined.
Thus each driving mechanism using a stepping motor needs a mechanism for determining an operation origin. In the conventional spectrophotometer shown in FIG. 9, the operation origins of the driving mechanisms (light source switching mechanism 4, filter selecting mechanism 7, and diffraction grating rotating mechanism 10) are detected by respective origin detecting sensors 5, 8, and 11. The origin detecting sensor is a photoelectric sensor or a microswitch, which detects an opening or a pin formed on a moving (rotating or shifting) part of the driving mechanism and determines the then position of the moving part as the operation origin of the stepping motor (or the driving mechanism). Such structure requires a detector for each of the three driving mechanisms, which causes various problems. Increase in the number of parts complicates the mechanisms and the whole control system. The precision and reliability of the detector itself need to be at the same high level as those needed for the spectrophotometer. These and other problems raise the cost of the spectrophotometer and lower the production efficiency.
When a photoelectric sensor is used as the origin detecting sensor, for example, many other auxiliary parts are necessary, including: a cable for connecting the sensor to a controller; a member for holding the sensor; a pin to be detected by the sensor; and a control circuit for transmitting signals from the sensor to a microcomputer. A microswitch also requires many auxiliary parts, and has poor reliability if used over a long time-period because of the mechanical contacts.
The reproducibility or precision of the operation origin depends on the precision of each origin detector. The precision of such sensors are generally not sufficiently high for general spectrophotometers which require high accuracy of setting the wavelength. In order to attain reproducibility or accuracy required to general spectrophotometers, costly high-quality sensors, complicated mechanism, and skilled adjustment are essential.